The present invention relates to a hollow golf club head, more particularly to a face portion having a variable thickness.
In the case of very large sized hollow golf club heads, e.g. wood-type golf club heads and the like, in order not to increase the weight of the club head, it is necessary to decrease the wall thickness of various portions as much as possible. If the thickness of the face portion is decreased, however, as the face portion receives a large impact force when hitting a ball, it is difficult to secure the required strength and durability.
Therefore, a countermeasure usually employed is as shown in FIG. 17, to decrease the thickness of an annular part surrounding the central part of the face portion in order not to decrease the strength of the central part.
In U.S. Patent application publication No. 2006-111201-A1, the rear face (a) of the face portion is as shown in FIG. 19, provided with relatively narrow, constant-width ribs (b). The ribs (b) extend radially from the face center so as to form reduced-thickness parts (c) between the ribs (b).
On the other hand, when the face portion hits a ball, large stress and large strain occur in the vicinity of the peripheral edge of the face portion. In the above two cases, as apparent from FIGS. 17 and 19, the reduced-thickness part, namely, thin part is formed along large portion of the peripheral edge of the face portion. Therefore, in view of the strength and durability, there is room for improvement. If the amount of decrease in the thickness of the reduced-thickness part is lessened, defeating the original purpose, then the deflection of the face portion at impact is decreased, therefore the rebound performance becomes liable to deteriorate.